The present invention relates to an optical element for an astable laser resonator which element may be employed as an output reflector and/or as a lens.
Heretofore, the output reflectors for such resonators had either a circular or a rectangular, more specifically a square cross section or configuration. However, theoretical considerations as, for example, described in IEE "Journal of Quantum Electronics", Volume QE-10, No. 3, pages 346 to 355, March 1974 have shown that such uniform or regular boundaries cause constructive interferences at the location of the virtual focus of the resonator. Such constructive interferences in turn cause non-homogeneous phase and amplitude profiles of the internal radiation field. Such interferences also increase the divergence of the output laser beam and they decrease the mode discrimination ratio which is a measure or indication for the operation of the laser, more specifically, whether the laser operates at all and how easily it operates in the phase mode of operation. Theoretically it can be said that undesired interferences may be diminished or entirely avoided if the reflector boundary which determines the beam decoupling or the beam output deviates in a predetermined manner from a regular contour. The simplest configurations or contours which deviate from a circular or a square contour are, for example, contours having an elliptical shape or a clover-leaf shape having two, three or a plurality of leaves or rosettes of a precisely given or predetermined shape. Thus, if one selects the reflector boundary in accordance with one of these theoretically calculated contours, it is possible to assure that the divergence of the output laser beam approximates the ideal minimum. In other words, the divergence approximates the divergence which is limited by diffraction.
Heretofore, the beam decoupling has been accomplished by means of a metal output reflector having a given radius of curvature which is determined by the resonator geometry and which has lateral dimensions smaller than the lateral dimensions of the radiation field in the resonator. These reflectors are convex full mirrors where the resonator is of the confocal type which is normally preferred. With this type of reflector, the beam decoupled from the laser corresponds to that proportion of the radiation field which extends outside the boundary of the reflector mirror.
Another prior art approach which under certain circumstances might be more advantageous, employs an auxiliary metallic mirror which is arranged in a slanted manner in the beam path ahead of the convex output reflector. This auxiliary mirror is normally a plain mirror provided with an aperture located concentrically in the beam path and having a smaller diameter than the diameter of the convex mirror or output reflector. This prior art arrangement corresponds, relative to the resonator geometry, precisely to the output reflector described above having the same diameter as the central opening of the plain mirror except that the output beam does not extend in the direction of the resonator axis but rather perpendicularly thereto. Heretofore, it was not possible to apply the above described theory because the production of the required reflectors was very expensive where these reflectors are high quality metal full mirrors with complicated boundary configurations as compared to circular or square, or rectangular boundaries. For example, it is economically unfeasible to produce the aperture with the required boundaries in metallic reflectors by means of spark erosion. Besides, spark erosion does not provide the required precision along the boundary configurations.
The production of such laser beam decoupling optical elements by other conventional means such as milling and grinding instead of spark erosion is also not feasible, because polishing must take place as the last production step whereby the precision of the boundary configuration is again diminished. The polishing is also made more difficult by the boundary edge of the aperture configuration. Thus, in any of these prior art methods of producing laser beam decoupling optical elements, it is not possible to produce a substantial number of such elements simultaneously in a batch type approach. The individual production is, thus, very expensive.
It is known to provide optical elements, for example for telescopes, with reflecting spots or holes, see U.S. Pat. No. 2,608,129 (Taylor). However, the boundaries of these spots or holes are circular and hence not suitable for achieving a diffraction limited divergence by means of a laser beam decoupling optical element.